Credibility Assessments in a Legal Context

Abstract

Lie detection is sometimes an easy task. A suspect who assures police officers that he has not spoken to his friend for several weeks, is definitely lying when CCTV footage caught him chatting with this friend just a few hours earlier. Detecting deceit is considerably more difficult when there is no physical or third-party evidence to rely on. In those situations, lie detectors have three options: (1) they can observe someone's behaviour, (2) analyse someone's speech, or (3) measure someone's physiological responses. Several credibility assessment tools designed to analyse speech and physiological responses have been developed, and the most well-known tools, Statement Validity Assessment (a verbal lie detection tool), the Control Question Test (a physiological lie detection test), and the Guilty Knowledge Test (another physiological lie detection test) will be introduced. I will briefly discuss how they work, how accurate they are, and to what extent they meet the criteria that are required for admitting expert scientific evidence in criminal courts according to the United States Supreme Court Daubert decision. Other verbal and physiological tools that are sometimes mentioned in the lie detection literature (Scientific Content Analysis technique (SCAN), Reality Monitoring (RM), Voice Stress Analysers (VSA), Relevant-Irrelevant Polygraph Test, and Directed Lie Polygraph Test), will also, albeit briefly, be touched on. I will also discuss nonverbal lie detection, despite the fact that nonverbal lie detection tools are virtually nonexistent. My main motive for discussing this is that nonverbal communication is a powerful source of information that can easily influence people's judgements, including those of solicitors, judges and jurors. A main issue in this chapter is how accurate the decisions are that lie detectors make (see Vrij in press for more information on all the lie detection tools mentioned in this chapter). Some comments about accuracy are therefore appropriate before discussing in detail the accuracy of several lie detection tests. I will begin with a definition of deception.